The invention described herein relates to nuclear reactor fuel assemblies and more particularly to an improvement in the design of control rod guide thimbles or tubes for minimizing control rod wear on the guide tubes during reactor operation.
Commercial nuclear reactor fuel assemblies include an array of fuel rods and control rod guide tubes held in spaced relationship with each other by grids of egg-crate configuration located at intervals along their length. Top and bottom nozzles are connected to the control rod guide tubes and these components together with grids form the structural framework for the assembly. The guide tubes principally provide a pathway for control rod insertion and withdrawal as the control rods are moved vertically to control reactor fissioning.
Each control rod guide tube is open at both ends to permit coolant to flow upwardly therethrough during reactor operation. Since the coolant flows at high velocity and pressure, it has the characteristic of inducing vibrations in the control rods, thus causing them to rapidly and cyclically engage the inner surface of their guide tubes. This action induces wear in the guide tubes, particularly in those areas where the tip of the control rod interfaces with the guide tube inner surfaces. The wear occurs circumferentially on the guide tube and it is evident that since the control rods are not moved a substantial distance over a relatively long time period, the wear could cause weakness of sufficient magnitude in each guide tube to substantially decrease its useful life.